bug#19060: [FIX INCLUDED] Off-by-one-line scrolling bug in window_scroll_pixel_based

[Kelly Dean]

With 24.4, emacs -Q, then M-x split-window-horizontally, then open any file 
with a lot of long logical lines (each a few visual lines long), so that as you 
page down through the file, every screenful of text has some wrapped lines on 
it. Adjust the vertical size of your Emacs frame so that the vertical size of 
your Emacs windows is not an integer multiple of the visual line height, so 
that the bottom visual line of text is partially covered by the top of the mode 
line.
Scroll forward a couple pages using pgdn (scroll-up-command), then notice 
exactly which visual line is at the top, and which one is at the bottom. Press 
pgdn, then pgup, and notice that the text doesn't return to the previous 
position; instead, it's scrolled backward by one visual line. This bug wasn't 
in 24.3.

The bug is in window.c of the 24.4 release, line 5042. The fix is to change 
⌜window_box_height (w)⌝ on that line to ⌜window_box_height (w) / dy * dy⌝ in 
order to align the result to an integer multiple of the visual line height, 
since as of 24.4, Emacs no longer ensures that window_box_height is such a 
multiple. Patch excluded, per request.

There might be a related bug (with the same fix) on line 4959, but I haven't 
tested that.

BTW on line 471 (copied from line 466) of window.h, ‟width” should be ‟height”.